A servovalve of the type described in German patent document No. 2,911,407 filed Mar. 23, 1979 by E. Brokoff and in descriptive literature RD 29,633/8.81 and RD 29,584 of the assignee of the instant application has a flow-control valve connectable between a source of fluid pressure and a load and having a spool displaceable between an open position permitting fluid flow between the load and the source and a closed position axially offset therefrom and blocking such flow. In a standard system the flow-control valve is a four-port three-position reversing valve. A pilot valve forming part of the servovalve has an impact plate and a pair of nozzles operatively axially oppositely effective on the spool and oppositely flanking the plate. Thus when the plate is closer to one of the nozzles than to the other the spool is axially displaced into one of its positions and when the plate is closer to the other nozzle the spool is urged axially oppositely. A solenoid-type servomotor is connected to the impact plate to move same between the nozzles so that when it is close to one nozzle it blocks flow therefrom and allows pressure to build up on the corresponding end of the spool, while the other nozzle is opened to reduce pressure at the corresponding spool end. A feedback spring is attached to the impact plate, extends generally radially of the spool, and has a free end projecting into a recess formed in the spool. A force-transmitting ball is fixed to the free end of the spring and fitted in the recess and has an outer surface engageable with the inner surface of the recess.
The feedback spring serves to urge the impact plate in a direction opposite to that the solenoid servomotor moves it. This feedback prevents the system from locking up and naturally returns it to a stable center position when the servomotor stops acting on the impact plate. In this center position all flow between the load and source is blocked.
Normally both the ball and spool are made of steel. It is therefore possible for them to corrode and stick together. In addition the sliding friction of steel on steel is considerable, so that this delicate force-transmitting coupling can freeze or fail to work accurately after a relatively short service life.